io6 PHYSIOGRAPHY. [chap. 



one, nor a supporter of combustion, like the other. When 

 two substances are simply mixed together, without entering 

 into chemical combination, they produce a mixture, having 

 properties which partake of those of its components. Thus 

 if four volumes of nitrogen are mixed with one volume of 

 oxygen, a mixture is obtained which resembles atmospheric 

 air, and is precisely what we should expect to produce ; the 

 activity of the oxygen being tempered by dilution with 

 nitrogen. For this, and other reasons, chemists believe that 

 atmospheric air is a mechanical jnixture of gases ; whilst 

 water is a true chemical compojind. 



In the methods hitherto described for the decomposition 

 of water, purely physical forces have been employed ; in the 

 one case it was electricity, in the other case heat. But a 

 similar decomposition may be effected by means of 

 chemical agencies. It has just been shown that water is a 

 compound of oxygen and hydrogen ; if, therefore, a body be 

 presented to it which has a very strong attraction for one of 

 these components, say the oxygen, it seems likely enough 

 that we shall be able to draw this away, and leave the other 

 constituent free. And such in truth is the case. Many of 

 the metals have powerful attraction for oxygen ; and, under 

 proper conditions, are capable of removing it from water, 

 and thus eliminating the hydrogen. There is, for example, 

 a metal well known to chemists as potassium, so called 

 in consequence of its existence in common " potashes." 

 Potassium so eagerly combines with oxygen that the 

 moment it is exposed to the atmosphere its surface becomes 

 covered with a film of oxide. Throw a small piece of 

 potassium upon water, and immediately a brilliant little 

 violet flame bursts forth upon the surface of the liquid, and 

 darts hither and thither until all the metal is spent. By this 

 means the water is broken up ; and the potassium has dis- 



